In conventional foam forming in the paper industry, for instance for web forming (wet laid techniques) of paper or foam coating, foam stability may be difficult to achieve. Surface active chemicals such as sodium dodecyl sulfate (SDS) can be used to achieve the desired stability of the foam, but these types of additives have often negative effects on the end product, i.e. reduced strength, and/or risks for migration which impairs the strength or barrier properties. Also some of these surface active chemicals are unwanted as they may not be allowed in food or liquid packaging materials.
Conventional foam forming is described in GB 1 395 757 where a foamed fiber dispersion is used in the manufacturing of paper. A surface active agent is added to the fibrous pulp, to provide a dispersion with an air content of at least 65%, to be placed onto the wire of the papermaking machine. The formation of wet laid webs have also been described in the Wiggins Teaoe Radofoam process where fibers were delivered to the wire of a conventional Fourdrinier machine in a suspension in aqueous form.
In more recent years the use of microfibrillated polysaccharide, such as microfibrillated cellulose (MFC), nanocellulose or nanocrystalline cellulose (NCC) has become increasingly interesting in the paper and board making process. However, the use of MFC has some drawbacks in that the barrier layer or film that is formed has a high drying shrinkage, as well as it is prone to absorbing more water than, for example, paper made from conventional pulp. The dewatering of films or barrier coating layers made from or comprising MFC are usually quite difficult, thus needing higher energy for dewatering, and reducing the efficiency and speed of the manufacturing process.
In WO2013/160564 A1 a method in which a hydrophobically sized layer of fibrous web is formed, is disclosed, by making a foam of water, a hydrophobic size, such as alkyl ketene dimer (AKD), a surfactant, such as oxylated C11-alcohol and microfibrillated cellulose.
There is a need for a method for forming a wet web, either by using a foam or conventional, or re-engineered wet laid techniques, which can be further dewatered and dried to a free-standing film or used as barrier coating having improved gas and aroma barrier properties at higher relative moisture content. The use of an amphiphilic starch in dispersion comprising microfibrillated fibers, offers further a barrier formulation that is substantially free from additives such as SDS.